This invention relates to tangential firing systems for use with pulverized solid fuel-fired furnaces, and more specifically, to an integrated low NO.sub.x tangential firing system, which is applicable to a wide range of solid fuels and which when employed with a pulverized solid fuel-fired furnace is capable of limiting NO.sub.x emissions therefrom to levels consistent with alternate solid fuel-based power generation technologies.
Pulverized solid fuel has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The tangential firing technique involves introducing the pulverized solid fuel and air into a furnace from the four corners thereof so that the pulverized solid fuel and air are directed tangent to an imaginary circle in the center of the furnace. This type of firing has many advantages, among them being good mixing of the pulverized solid fuel and the air, stable flame conditions, and long residence time of the combustion gases in the furnaces.
Recently though, more and more emphasis has been placed on the minimization as much as possible of air pollution. In this connection, with reference in particular to the matter of NO.sub.x control it is known that oxides of nitrogen are created during fossil fuel combustion primarily by two separate mechanisms which have been identified to be thermal NO.sub.x and fuel NO.sub.x Thermal NO.sub.x results from the thermal fixation of molecular nitrogen and oxygen in the combustion air. The rate of formation of thermal NO.sub.x is extremely sensitive to local flame temperature and somewhat less so to local concentration of oxygen. Virtually all thermal NO.sub.x is formed at the region of the flame which is at the highest temperature. The thermal NO.sub.x concentration is subsequently "frozen" at the level prevailing in the high temperature region by the thermal quenching of the combustion gases. The flue gas thermal NO.sub.x concentrations are, therefore, between the equilibrium level characteristic of the peak flame temperature and the equilibrium level at the flue gas temperature.
On the other hand, fuel NO.sub.x derives from the oxidation of organically bound nitrogen in certain fossil fuels such as coal and heavy oil. The formation rate of fuel NO.sub.x is strongly affected by the rate of mixing of the fossil fuel and air stream in general, and by the local oxygen concentration in particular. However, the flue gas NO.sub.x concentration due to fuel nitrogen is typically only a fraction, e.g., 20 to 60 percent, of the level which would result from complete oxidation of all nitrogen in the fossil fuel. From the preceding it should thus now be readily apparent that overall NO.sub.x formation is a function both of local oxygen levels and of peak flame temperatures.
Over the years, there have been numerous modifications made to the standard tangential firing technique. Many of these modifications, and in particular those that have been suggested most recently, have been proposed primarily in the interest of achieving an even better reduction of emissions through the use thereof. The resultant of one such modification is the firing system that forms the subject matter of U.S. Pat. No. 5,020,454 entitled "Clustered Concentric Tangential Firing System", which issued on Jun. 4, 1991 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,020,454, there is provided a clustered concentric tangential firing system that is particularly suited for use in fossil fuel-fired furnaces. The clustered concentric tangential firing system includes a windbox. A first cluster of fuel nozzles are mounted in the windbox and are operative for injecting clustered fuel into the furnace so as to thereby create a first fuel-rich zone therewithin. A second cluster of fuel nozzles are mounted in the windbox and are operative for injecting clustered fuel into the furnace so as to thereby create a second fuel-rich zone therewithin. An offset air nozzle is mounted in the windbox and is operative for injecting offset air into the furnace such that the offset air is directed away from the clustered fuel injected into the furnace and towards the walls of the furnace. A close coupled overfire air nozzle is mounted in the windbox and is operative for injecting close coupled overfire air into the furnace. A separated overfire air nozzle is mounted within the burner region of the furnace so as to be spaced from the close coupled overfire air nozzle and so as to be substantially aligned with the longitudinal axis of the windbox. The separated overfire air nozzle is operative for injecting separated overfire air into the furnace.
The resultant of another such modification is the firing system that forms the subject matter of U.S. Pat. No. 5,146,858, which is entitled "Boiler Furnace Combustion System" and which issued on Sep. 15, 1992. In accordance with the teachings of U.S. Pat. No. 5,146,858, a boiler furnace combustion system is provided of the type that typically includes main burners disposed on side walls of or at corners of a square-barrel-shaped boiler furnace having a vertical axis with the burner axes being directed tangentially to an imaginary cylindrical surface coaxial to the furnace. Moreover, in this type of boiler furnace combustion system air nozzles are disposed in the boiler furnace at a level above the main burners so that unburnt fuel left in a reducing atmosphere or a lower oxygen concentration atmosphere of a main burner combustion region can be perfectly burnt by additional air blown through the air nozzles. The boiler furnace combustion system, as taught in U.S. Pat. No. 5,146,858, is particularly characterized in that two groups of air nozzles are disposed at higher and lower levels, respectively. More specifically, the air nozzles at the lower level are provided at the corners of the boiler furnace with their axes directed tangentially to a second imaginary coaxial cylindrical surface having a larger diameter than the first imaginary coaxial cylindrical surface. The air nozzles at the higher level, on the other hand, are provided at the centers of the side wall surfaces of the boiler furnace with their axes directed tangentially to a third imaginary coaxial cylindrical surface having a smaller diameter than the second imaginary coaxial cylindrical surface.
The resultant of yet another such modification is the firing system that forms the subject matter of U.S. Pat. No. 5,195,450 entitled "Advanced Overfire Air System for NO.sub.x Control", which issued on Mar. 23, 1993 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,195,450, there is provided an advanced overfire air system for NO.sub.x control, which is designed for use in a firing system of the type that is particularly suited for use in fossil fuel-fired furnaces. The advanced overfire air system for NO.sub.x control includes multi-elevations of overfire air compartments consisting of a plurality of close coupled overfire air compartments and a plurality of separated overfire air compartments. The close coupled overfire air compartments are supported at a first elevation in the furnace and the separated overfire air compartments are supported at a second elevation in the furnace so as to be spaced from but aligned with the close coupled overfire air compartments. Overfire air is supplied to both the close coupled overfire air compartments and the separated overfire air compartments such that there is a predetermined most favorable distribution of overfire air therebetween, such that the overfire air exiting from the separated overfire air compartments establishes a horizontal "spray" or "fan" distribution of overfire air over the plan area of the furnace, and such that the overfire air exits from the separated overfire air compartments at velocities significantly higher than the velocities employed heretofore.
Throughout the 1990s and into the twenty-first century large, central pulverized solid fuel-fired power stations are expected to play an important role in worldwide power generation. These stations will be designed for maximum cycle efficiency, multiple-fuel flexibility, cycling, maximum availability, least capital cost, minimum maintenance cost, and lowest possible emissions that meet or exceed federal, state and local rules. Historically, tangential firing has demonstrated inherently low NO.sub.x production for large, pulverized solid fuel-fired furnaces. Lower NO.sub.x emissions result from the staging that occurs with the physical separation of the pulverized solid fuel and air streams emanating from the corner windboxes. The flames produced at each pulverized solid fuel nozzle are stabilized through global heat- and mass-transfer processes. A single rotating flame envelope ("fireball"), centrally located in the furnace, provides gradual but thorough and uniform pulverized solid fuel-air mixing throughout the entire furnace. This tangential firing process has been an advantage in developing advanced air staging systems for combustion NO.sub.x control. In contrast, wall-fired furnaces utilize groups of individually self-stabilizing burners that do not use global furnace flow patterns to achieve uniform pulverized solid fuel and air mixing. As a result, wall-fired arrangements, even though employing separated overfire air, typically create local zones of high temperature and O.sub.2 concentrations that cause NO.sub.x formation.
Thus, although firing systems constructed in accordance with the teachings of the three issued U.S. patents to which reference has been made hereinbefore have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for such firing systems to be improved. More specifically, a need has been evidenced in the prior art for a new and improved tangential firing system that would enable NO.sub.x emissions from pulverized solid fuel-fired furnaces to be controlled at levels, which are consistent with alternate pulverized solid fuel-based power generation technologies, such as circulating fluidized bed (CFB) and integrated gasification combined cycle (IGCC), without utilizing either selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR). To this end, a need has been evidenced in the prior art for a new and improved tangential firing system that would enable the NO.sub.x emissions from pulverized solid fuel-fired furnaces to be limited to less than 0.15 lb./10.sup.6 BTU, while yet at the same time limiting carbon-in-flyash to less than 5% and CO emissions to less than 50 ppm. Moreover, such emissions levels should be attainable while a wide range of solid fuels, from medium-volatile bituminous coal through lignite, are being fired in a pulverized solid fuel-fired furnace that has been equipped with such a new and improved tangential firing system. Finally, there is a need in order that such a new and improved tangential firing system may be provided that attention be focused on the entire pulverized solid fuel combustion system, including pulverization, primary air flow, fuel admission assemblies, and multiple levels of air injection (auxiliary air, close-coupled overfire air, and separated overfire air). To this end, such a new and improved tangential firing system may be viewed as consisting of the following four major elements: solid fuel pulverization and classification, pulverized solid fuel admission and combustion near the pulverized solid fuel nozzle tip, lower furnace combustion, and upper furnace combustion (between the main windbox and the furnace arch). Moreover, such a new and improved tangential firing system should be predicated on the optimization therewithin of these four above-enumerated individual elements.
To thus summarize, a need has been evidenced in the prior art for a new and improved tangential firing system that when employed with a pulverized solid fuel-fired furnace is capable of meeting 0.10 to 0.15 lb./10.sup.6 BTU NO.sub.x emissions levels on Eastern U.S. bituminous coals, and of making pulverized solid fuel firing in a pulverized solid fuel-fired furnace competitive on an emissions basis with other new solid fuel-fired technology options, such as fluidized bed combustors and IGCC. Moreover, with such a new and improved tangential firing system the NO.sub.x emission target is to be achieved through combustion techniques only, while maintaining carbon-in-flyash at less than 5% and CO emissions at less than 50 ppm. That is, such a new and improved tangential firing system should be capable of enabling minimum total emissions to be achieved therewith. In this regard, techniques employed to reduce NO.sub.x formation, such as sub-stoichiometric primary zone combustion, staging of pulverized solid fuel and air mixing, reduced excess air, and lower heat release rates, are all aimed at controlling oxygen availability, the combustion rate and reducing peak flame temperatures. However, since these conditions may increase the potential for CO, hydrocarbons, and increased unburned carbon emissions, it is necessary that in such a new and improved tangential firing system that a balance be achieved among these opposing factors. Namely, it is necessary that such a new and improved tangential firing system comprise an integrated tangential firing system wherein finer solid fuel pulverization is combined with advanced pulverized solid fuel admission assemblies and in-furnace air staging utilizing multiple air injection levels. It is the integration of these features, which distinguishes such a new and improved integrated tangential firing system from prior art forms of firing systems.
The need for finer solid fuel pulverization is predicated on the need to minimize combustible losses (unburned carbon) caused by the staged combustion process for NO.sub.x control. Finer pulverized solid fuel can result in close ignition at the pulverized solid fuel nozzle tip discharge, enhancing fuel-bound nitrogen release and its subsequent reduction to N.sub.2 under staged conditions. Secondary benefits include fewer large (&gt;100 mesh) particles impinging on the waterwalls of the pulverized solid fuel-fired furnace and improved low-load ignition stability.
The need for advanced pulverized solid fuel admission assemblies is to ensure that the ignition point of the pulverized solid fuel occurs closer to the nozzle tip than it does with conventional pulverized solid fuel nozzle tips. The rapid ignition of the pulverized solid fuel produces a stable volatile matter flame and minimizes NO.sub.x production in the pulverized solid fuel-rich stream. In addition, there should also exist the capability with the advanced pulverized solid fuel admission assemblies to horizontally offset some of the windbox secondary airflow in order to thereby make less air available to the pulverized solid fuel stream during the early stages of combustion. Such horizontally offsetting of some of the windbox secondary airflow also creates an oxidizing environment near the waterwalls of the pulverized solid fuel-fired furnace in and above the firing zone. This reduces ash deposition quantity and tenacity and results in both less wall soot blower usage and increased lower furnace heat absorption. Increased O.sub.2 levels along the waterwalls of the pulverized solid fuel-fired furnace also reduce corrosion potential, especially when coals with high concentrations of sulfur, iron, or alkali metals (K, Na) are fired. Corrosion by sulfidation or other mechanism(s) can be largely controlled in practice by minimizing the potential for direct fuel impingement on the waterwalls of the pulverized solid fuel-fired furnace. This potential is addressed via conservative heat release parameters and pulverized solid fuel-fired furnace geometries, as well as improved pulverized solid fuel fineness control.
The need for in-furnace air staging utilizing multiple air injection levels is predicated on the need to discharge a portion of the secondary air through air compartments at the top of the main windbox to improve carbon burnout without increasing NO.sub.x production. In addition, there should also exist the capability with the in-furnace air staging utilizing multiple air injection levels to control firing zone stoichiometry through multi-staged separated overfire air (SOFA). Two or more discrete levels of overfire air are incorporated in the corners of the pulverized solid fuel-fired furnace between the top of the main windbox and the pulverized solid fuel-fired furnace outlet plane to create the optimum stoichiometry history for NO.sub.x control for a given pulverized solid fuel. The SOFA compartments have adjustable yaw and tilt positioning, which allows tuning of the combustion air and pulverized solid fuel-fired furnace gas mixing process for maximum control of combustible emissions such as carbon, CO, total hydrocarbons (THC) and polycyclic aromatic compounds (PAC).
It is, therefore, an object of the present invention to provide a new and improved tangential firing system that is particularly suited for use with pulverized solid fuel-fired furnaces.
It is a further object of the present invention to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that through the use thereof NO.sub.x emissions from pulverized solid fuel-fired furnaces can be controlled at levels, which are consistent with alternate pulverized solid fuel-based power generation technologies, such as circulating fluidized bed (CFB) and integrated gasification combined cycle (IGCC), without utilizing either selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR).
It is another object of the present invention to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that through the use thereof NO.sub.x emissions from pulverized solid fuel-fired furnaces can be less than 0.15 lb./10.sup.6 BTU.
It is still another object of the present invention to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that through the use thereof NO.sub.x emissions from pulverized solid fuel-fired furnaces can be limited to less than 0.15 lb./10.sup.6 BTU while yet at the same time limiting carbon-in-flyash to less than 5% and CO emissions to less than 50 ppm.
Another object of the present invention is to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that through the use thereof NO.sub.x emissions from pulverized solid fuel-fired furnaces can be limited to less than 0.15 lb./10.sup.6 BTU while a wide range of solid fuels, from medium-volatile bituminous coal through lignite, are being fired in the pulverized solid fuel-fired furnace.
A still another object of the present invention is to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that included therewithin as an element thereof is solid fuel pulverization and classification.
A further object of the present invention is to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that included therewithin as an element thereof is pulverized solid fuel admission and combustion near the pulverized solid fuel nozzle tip.
A still further object of the present invention is to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that included therewithin as an element thereof is lower furnace combustion.
Yet an object of the present invention is to provide such a new and improved tangential firing system for pulverized, solid fuel-fired furnaces which is characterized in that included therewithin as an element thereof is upper furnace combustion.
Yet a further object of the present invention is to provide such a new and improved tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that finer solid fuel pulverization is combined therewithin with advanced pulverized solid fuel admission assemblies and in-furnace air staging utilizing multiple air injection levels such that the new and improved tangential firing system thereby constitutes a new and improved integrated tangential firing system for pulverized solid fuel-fired furnaces.
Yet another object of the present invention is to provide such a new and improved integrated tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that it is equally well suited for use in either new applications or in retrofit applications.
Yet still another object of the present invention is to provide such a new and improved integrated tangential firing system for pulverized solid fuel-fired furnaces which is characterized in that it is relatively easy to install, relatively simple to operate, yet is relatively inexpensive to provide.